Back of the knee usually means saddle to high. If you’ve been in this saddle position for some time its more likely that you’ve damaged the knee off the bike and the pedalling action agravates it.
Go see a physio first and then get to a bike fitter to make sure you’re set up is right.
email me if you want my address is in my profile
I did try spds on my mtb a few weeks ago which is what I think masy have aggravated it. The cleats were set up differently to how the are on my road bike and I pedalled with a slightly different technique to account for the lower saddle height. However, I havent ridden ym mtb since, just my road bike.
Whats the best thing to do? Time off? Its not a massive discomfort, I managed about a 7 hour ride on saturday and it didnt get any worse, its just annoying.